Can I use my IFS App for tagging contacts coming from two website?

Hi we have a membership website that gives access to video content. We are expanding that content to a different but related niche. That means we will be creating a new website - so a different door, to give this related niche access to the same content. So my question is - how do I use the api on what appears to be a different website? I know I will simply be tagging those people as being from the different niche but how do I actually make it happen via api and wp fusion etc?